The production of hydrocarbon mixtures, in particular also of short-chain olefins, by conversion of oxygenates by using form-selective molecular sieve catalysts, in particular of pentasil zeolites of the structure type ZSM-5, is known from the prior art and described for example in the European Patent Application EP 0448000 A1 and the European Patent Specification EP 1289912 B1. The use of multi-stage fixed-bed reactors for this purpose also has been described already. For example, the International Patent Application WO 96/15082 A1 teaches a process for converting a feed mixture containing oxygenate compounds, for example methanol or dimethyl ether, into gasoline-like hydrocarbon compounds in a multi-stage fixed-bed process. In this process, fresh feed material containing oxygenates is supplied to a reaction zone together with the product gas from a preceding reaction zone and additional dilution gas. The temperature and composition of the dilution gas is chosen such that the increase in temperature in the exothermal reaction of the oxygenates to hydrocarbons in each of the succeeding reaction zones is limited to a maximum of 150° C., wherein the steam partial pressure should not exceed 2.2 ata. In this way, a premature deactivation of the zeolite catalyst used should be prevented, since too high a steam partial pressure at too high temperatures leads to an irreversible change in structure of the zeolite, with which catalytically active centers get lost. On the other hand, the steam is required as dilution medium and to prevent excessive carbon deposits on the catalyst. A slow deposition of carbon on the catalyst during the synthesis operation, however, is inevitable. When the same exceeds a tolerable maximum, the production operation must be interrupted and the carbon deposits must be removed for example by controlled burning off. The catalytic activity of the catalyst thereby can largely be restored, i.e. regenerated. The regeneration of the catalyst can be repeated several times, until the above-described irreversible deactivation has decreased the catalytic activity so much that a further use of the catalyst is prohibited for economic reasons. The time interval with production operation of the catalyst between two regenerations is referred to as cycle or also reaction cycle. The first cycle is the operating phase between the restart of the reactor with newly produced catalyst and the first regeneration.
The International Patent Application WO 2007/140844 A1 relates to a reactor for producing C2 to C8 olefins, preferably propylene, from a feed mixture comprising gaseous oxygenate, preferably dimethyl ether (DME) and/or methanol, steam and one or more of the hydrocarbons, which has a temperature of 400 to 470° C., and to a method for operating the reactor. The reactor contains a plurality of reaction stages or reaction zones arranged inside a closed upright container, which are traversed by the material stream from the top to the bottom, each consisting of a supporting tray with a fixed-bed zone located thereon, which is formed of a bed of granular molecular sieve catalyst. In a particular configuration, the reactor contains six reaction zones. Each supporting tray is constructed of cells firmly connected with each other, which are arranged one beside the other without spaces, and is suspended freely in the container. The cells are filled with a layer of molecular sieve catalyst. In the space defined by two adjacent reaction zones at the top and at the bottom, an atomizer system each is provided in the form of a number of nozzle tubes for uniformly spraying a liquid phase containing DME and/or methanol, chiefly consisting of steam and having a temperature of 25 to 150° C. by means of a gas phase saturated with water, chiefly containing DME and/or methanol and having a temperature of 170 to 300° C. towards the reaction stage following next in downstream direction. By spraying the liquid phase, the temperature of the reaction mixture exiting from the reaction stage with a temperature of 400 to 500° C. is lowered to a value of 380 to 470° C., so that the reaction proceeds in a narrow temperature range (quasi isothermally). The liquid phase can contain up to 30 vol-% of DME and/or methanol and the gas phase can contain up to 80 vol-% of DME and up to 30 vol-% of methanol.
For operating the reactor, a feed mixture containing gaseous oxygenate, preferably DME and/or methanol, as well as steam, which has a temperature of 150 to 300° C., is cooled to a temperature of 100 to 160° C., separated into a liquid phase and a gas phase, and liquid phase and gas phase are divided into several partial streams whose number each corresponds to the number of the spaces existing between the reaction stages. Based on a space, a gas-phase partial stream after heating to a temperature of 170 to 300° C. and a liquid-phase partial stream after cooling to a temperature of 25 to 150° C. each is supplied to an atomizer and sprayed into the space. By supplying gas and liquid in a corresponding temperature and quantity between the individual reaction stages, the inlet temperature of the reaction mixture exiting from the reaction stage into the space can be adjusted to the desired temperature before entry into the next following reaction stage.
The features of the reactor described in the document WO 2007/140844 A1 according to claims 1 to 13 as well as the features of the method for operating the reactor according to claims 14 to 15 and furthermore the description of an exemplary embodiment according to FIG. 1 to FIG. 7 and the associated description of Figures on p. 5 to p. 8 herewith are incorporated into the disclosure of the present patent application by reference.
The International Patent Application WO 2010066339 A1 teaches a process for producing a product containing propylene and ethylene by converting methanol and ethanol at the same time in an adiabatic reactor containing a plurality of series-connected reaction zones, wherein each reaction zone is covered with a fixed bed of form-selective catalyst, in that a feed mixture, comprising gaseous methanol, DME, steam and possibly one or more C2, C4, C5, C6, C7, C8 olefins and paraffins, is charged at least to the first reaction stage of the reactor at temperatures of 300 to 600° C. and pressures of 0.1 to 20 bar, absolute. By the distributed addition of the feed mixture containing oxygenates, advantages are achieved with regard to the temperature control and the suppression of undesired consecutive and side reactions.
With reference to the prior art discussed above, it can clearly be seen that so far it has been the primary objective to design the process of producing short-chain olefins from oxygenates by means of a multi-stage reactor such that optimum yields are obtained for the target products, i.e. in particular ethylene and propylene, with a good control of the exothermicity of the conversion reaction, since a maximization of the yield represents an important parameter for optimizing the process economy. What has been considered less, however, is the influence of the duration of the reaction cycles on the economy of the production process.
The catalyst life achieved so far with the production processes known from the prior art as described above is regarded as comparatively short—in particular with regard to the considerable price of the catalyst. Furthermore, several regenerations are required to achieve this previous maximum catalyst life.
Carrying out the regeneration between the various reaction cycles leads to a reduction of the operating period per reactor and per operating year and to an increased consumption of operating materials. This applies in particular to production plants which are operated outside an integrated association with ancillary facilities supplying operating materials. Carrying out each individual step of the regeneration procedure and transferring the reactor from the operating state into the regeneration mode and back binds a considerable part of the operating costs in the plant. In addition, the reactor is not available for the olefin production during the regeneration, so that the maintenance of a continuous production operation requires a multi strand concept for the production plant. Therefore, an increased catalyst life and an increase in the number of reaction cycles (tantamount to a reduced number of regenerations) would considerably improve the process economy of the production of short chain olefins from oxygenates.